1. Field of the Invention
The present invention relates to temperature measurement using pyrometry, and more specifically, it relates to the measurement of temperature and emissivity using single-fiber multi-color pyrometry.
2. Description of Related Art
Radiation thermometry is a common non-contact method of measuring temperature. Planck's Law states that the spectral radiance of a target is a function of its temperature. Hence, the signal produced by a detector that is sensitive to all or part of the radiated thermal spectrum will be related to the temperature of the target. However, the spectral radiance of a target is also governed by its emissivity. Consequently, the signal will depend on the emissivity of the target as well as its temperature. Furthermore, the measured radiance may be comprised of unwanted ambient system radiance as well as the desired target radiance, and may be weak compared to the detector background level.
Currently, many temperature sensing devices employ the method of two-color pyrometry to eliminate the effect of unknown or varying emissivity. Two-color pyrometers sample the target radiance in two different spectral regions, and calculate the true temperature and/or emissivity using various algorithms. Several techniques for separating the target radiance into two spectral regions have been identified. One technique involves a beamsplitter to direct the incident radiation into two paths, each of which contains a detector. A second method incorporates a rotating filter wheel composed of two different filters and a single detector. Another method uses a two-color detector consisting of two different active regions. As with any radiation thermometer, the spectral characteristics of the optical components determine the useful temperature range of the device.
The systems developed by X. Maldague, et. al. [Opt. Eng. 28(8):872-80] and U. Anselmi-Tamburini, et. al. [Rev. Sci. Instr. 66(10):5006-10] both employ beamsplitters to separate the incident radiation into two paths, each of which contains a detector. The device patented by K. Crane, et. al. [U.S. Pat. No. 4,470,710] employs a wheel composed of alternating infrared filters of different bandpass and a single detector. These systems are suited only to high-temperature measurement. High-temperature measurement methods require large target signals that are generally much stronger that any ambient radiation. In general, simply replacing the optical components in high-temperature devices with longer wavelength components will not provide a clear signal for low-temperature measurement, because the necessary means of distinguishing the small target signal from the ambient noise is missing.
The system developed by O. Eyal and A. Katzir [Opt. Eng. 34(2):470-3] exhibits state-of-the-art technology for remote low-temperature two-color pyrometry. A single silver halide optical fiber collects radiation emitted by a target and transmits it to an optical chopper which modulates the radiation for lock-in amplification before it is focused onto a single two-color mid-infrared detector. The side of the chopper facing the detector is made reflective to stabilize the lock-in signals by one of two methods: either a reference blackbody of controlled temperature is positioned such that the detector alternately "sees" it and the target, or a black line is drawn on the chopper blades to control its emissivity. This device offers several features. First, the use of a single collection fiber ensures that each spectral region is comprised of radiation emitted by the same spot on the target, which, when coupled with the two-color principle, minimizes the influence of the area of the spot (i.e. fiber tip to target distance). Second, lock-in amplification enables recovery of small signals generated by low-temperature targets. Third, the reflective chopper provides a means of lock-in signal referencing. Fourth, the two-color mid-infrared detector incorporates the two active regions in a single element. If the spectral sensitivities of the optical components were chosen differently, high-temperature measurements could theoretically be made using the same method.
There is a need to perform color-temperature measurements using a fiber-based system in which the detected radiation is collected by a single fiber, and the radiation is detected in two or more wavelength bands. Single fiber collection eliminates the need to align multiple fibers to a common spot on the target. The method of Eyal and Katzir allows such a measurement technique using an integrated two-color detector system, but its extension to multiple wavelength bands is limited by detector technology. The present invention may be extended to multiple bands, and does not rely on sophisticated detector arrangements. Like the above low-temperature device, this method uses a single optical fiber to ensure that the radiation collected in each spectral band originates from the same spot on the target.